Use PVC. It will hold the cold better. And insulate it with Armaflex or something similar. You might be able to find it at Home Depot or a plumbing supply house.
You might look into finding some flexible A/C duct. It is already insulated & can be found in 3" or 4" dia. if you look hard enough.
I would think the 4" tower would allow better air flow.
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